1. Field of the Invention
The invention relates to a method for measuring time-variant magnetic fields using magnetoelectric sensors.
2. Description of the Related Art
Magnetoelectric sensors, also referred to as ME sensors, are among others suitable for detecting small time-variant magnetic fields that are caused for example by currents in biological organisms. They are regarded as promising candidates for replacing so-called SQUIDS that are based on superconductivity and for this purpose require constant and extreme cooling. ME sensors are the subject matter of current research among others with respect to the development of biomagnetic interfaces that might be used at first in medical diagnostics, e.g. MEG, MCG, and in future possibly also on prosthesis control or even for the general ‘thought control’ of computers and machines.
The basic concept of the functioning of all ME sensors is the mechanical force coupling of magnetostrictive and piezoelectric materials.
Magnetostrictive materials, e.g. ferromagnetic transition metals, Fe, Ni, Co, and their alloys, compounds of rare earths Tb, Dy, Sm with the ferromagnetic transition metals, e.g. TbFe2, SmFe2, or also ferromagnetic glasses that predominantly contain the elements iron, cobalt, boron or silicon in varying quantities, experience a reversible change in length in the direction of a magnetic field that acts on them. This change in length is attributed to the orientation of elementary magnets along the external magnetic fields and according to present knowledge can amount to up to 2.5 mm/m=2500 ppm at room temperature.
If a magnetostrictive material is now coupled mechanically firmly to a piezoelectric, e.g. lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF), aluminum nitride (AlN), the magnetostrictive expansion can exert a force that leads to a structural charge transfer, polarization, in the piezoelectric, that again leads to a measurable piezo voltage. This voltage can be detected electronically as a measure for the magnetic field strength and evaluated.
There is a diversity of ME sensors of different designs. Among the most simple is a multi-layer film system comprising at least one layer from a magnetostrictive material directly having arranged thereon a piezoelectric layer and a metallization layer as electrode on the piezo material. Conventionally, the film system is in the shape of a strip that is attached at at least one end. With a magnetic field acting along the strip length, the strip bends due to the different expansion of the material, and the piezo material which is thus simultaneously bent is polarized electrically. The electric potential difference between the two flat strip sides can be tapped as the measurement voltage.
Magnetostrictive, also referred as to ME, and piezoelectric, also referred as to PE, material films can be deposited on top of each other and/or on predetermined substrates using coating methods that are known per se. The manufacture of ME sensors is to this extent compatible with processes of the silicon technology; in particular integrated ME sensors can be manufactured for example in MEMS style, Micro Electrical Mechanical Systems. However, the separate production of MS and PE foils and subsequently gluing both together to form an ME foil is suited for producing magnetic field sensors according to the principle described, too.
All ME sensors are mechanical oscillators. When a periodic magnetic field of defined frequency acts on them, they exhibit a forced mechanical oscillation behavior. If in the process the excitation takes place at the mechanical resonant frequency of the ME sensor, even very small magnetic field strengths result in very great measurement voltages.
Biologically produced magnetic fields typically only have frequencies of the order of magnitude of 1 Hz up to approximately 100 Hz. In contrast, the resonant frequencies of common ME sensors amount to some 100 Hz up to a few 100 kHz. It can be expected that a further miniaturization of the ME sensors, for example by integration into MEMS, may further lead to even higher resonant frequencies.
The ratio between the ME electric field-strength amplitude caused in the ME sensor and the exciting magnetic field-strength amplitude is referred to as ME coefficient αME. The ME coefficient typically varies by two to three orders of magnitude between measurements of magnetic fields in the resonant case and far outside the resonance.
It would therefore be desirable to be able to have at one's disposable in each case ME sensors having a suitable resonant frequency, in order to detect small magnetic fields of a frequency known in advance. In fact there are efforts to tune for example ME sensors of the strip style described initially to lower resonant frequencies by arranging additional masses. However, the ME coefficient even then exhibits a sharply limited maximum at the resonant frequency so that adjacent frequencies provide markedly weaker signals. Even an array of ME sensors that all exhibit different resonant frequencies, e.g. proposed in US 2010/0015918 A1, the ME sensors being intended as receivers of magnetic carrier waves and the array realizing a multiplicity of data channels, does not necessarily lead to a sufficiently dense scanning of a frequency band on which an a priori unknown signal is to be detected. Over and above this, an array having hundreds of ME sensors could in practice only be manufactured as an integrated microsystem, and reducing the mechanical resonant frequencies of kHz oscillators to the biomagnetic band range, 100 Hz, can hardly be carried out in the process.
Alan S. Edelstein, et al. “Approach for sub pT, Room Temperature Magnetic Sensors” Sensors, 2010 IEEE, IEEE, Piscataway, N.J., USA dated 1 Nov. 2010, pages 620-622, discloses a method for measuring a time-variant magnetic field using a magnetoelectric sensor, where the magnetic field—in the paper a test field having a frequency of 10 Hz is used—is modulated using a rotating disk that acts as a ‘magnetic flux concentrator’, to suppress the 1/f noise. Modulation takes place at the rotational frequency of the disk, 76 Hz, and causes a frequency conversion of the magnetic field, and thus of the ME sensor signal, to 66 and 86 Hz. Edelstein et al. also propose to carry out the frequency conversion up to and into the mechanical resonant frequency of the ME sensor. Since, however, this is positioned between 200 and 300 kHz according to FIG. 2 shown there, use of a rotating disk as magnetic flux concentrator in principle does not seem to be suitable for this purpose.
Greve, Henry et al. “Giant magnetoelectric coefficients in (Fe90Co10)78Si12B10-AlN thin film composites”, Applied Physics Letters, AIP, American Institute of Physics, Melville, N.Y., USA, dated 3 May 2010, pages 182501-182501, further discloses a thin film magnetoelectric composite, using which it seems possible to realize a 3-dimensional vector field sensor since a sensor element having a preferred sensitivity only in one dimension could be manufactured.
ME sensors are usually also operated outside their mechanical resonance. Since an interest exists in the greatest possible measurement dynamics, the greatest possible signal noise ratio, and the linearity of the voltage response to the alternating magnetic field that is to be measured, an operating point for the ME sensor is selected in the linear domain of the magnetostriction characteristic, see FIG. 1. This characteristic λ (H) describes the length expansion λ of the MS material under the influence of a magnetic field H and its course is always symmetrical since both field directions have the same effect on the material. Without a field, the function λ (H) initially has a parabolic rise, but at the same time has an upper limit, on reaching the saturation magnetization. Consequently, it exhibits an inflection point HB where the linear term of the Taylor expansion of λ dominates around H=HB and where at the same time the greatest gradient occurs. To operate the ME sensor in this favorable working point, preferably a constant magnetic bias field having the strength HB is applied by suitably arranging current conductors or permanent magnets.
However, such a magnetic bias field is not without problems when applied in practice. Especially in the case of several ME sensors in a very close neighborhood relative to each other, mutual influences of the magnetic bias fields can occur, in particular if the fields are generated according to the Biot-Savart Law and the currents have to be conducted via leads. In terms of energy, permanent-magnetic bias fields are more favorable, but simply require a sufficient amount of magnetic material to be arranged so as to obtain suitable field strengths. Great effort is therefore placed on developing film systems and laminates from magnetostrictive and piezoelectric materials for ME sensors that can be brought into the operating point using the smallest possible magnetic bias field strengths. Such film systems are described for example in the paper by Zhai et al. “Giant magnetoelectric effect in Metglas/polyvinylidene-fluoride laminates”, APPLIED PHYSICS LETTERS 89, 083507 (2006) and in U.S. Pat. No. 7,023,206 B2. Nevertheless, a bias field of several Oersted, obsolete cgs unit for the magnetic field strength: 1 Tesla=μ0×10,000 Oersted, is still required.